Thursday, July 08, 1999 › SampleReport.doc · Web viewHysteresis Check. Found Acceptable values Hysteresis 0.528 % less than 1% . from 1 % to 2% more than 3% ideal acceptable

Saturday, May 20, 2023

Company nameCompany address

RE/ Process control loop analysis Loop-Tag

Dear Sir,

We submit this report on the loop Loop-Tag. This analysis should help you improve the performance, organize maintenance and update your information.

Please call us with questions.

Regards,

ExperTune Inc. document.doc Page 1 of 1

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Initial Tests

Automatic Mode

Manual mode

Notes:

ExperTune Inc.document.doc Page 1 of 18

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Hysteresis Check

Found Acceptable valuesHysteresis 0.528 % less than 1%

from 1 % to 2%more than 3%

idealacceptable

to be checkedGain 0.822 less than 0.5

from 0.5 to 2more than 3

too smallideal

too highNoise 0.873 % less than 3% acceptableValve:

Note: Enter your valve recommendations here.

If your hysteresis is more than 1% for valves with positioners or 3% for valves without positioners you should consider repairing or changing equipment to reduce hysteresis and improve control. Often, the addition of a valve positioner will correct the problem.

With a sticky valve, the air signal to the valve will have to change by an amount equal to the hysteresis before the valve stem will move. Once the valve has begun to move in one direction it will continue to move if the air signal keeps moving in the same direction. When the air signal reverses direction, the valve will not move until the air signal has changed in the new direction by an amount equal to the hysteresis.

Hysteresis is not limited to valves, it is also present in mechanical linkages, and can have other sources, but valves are the most typical sources.


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Stiction Check

Found Acceptable valuesStiction Stiction is less than 1% less than .2%

more than .3%ideal

to be checkedCO High 61CO Low 60Valve:

Note: Enter your valve recommendations here.

In many processes, 0.5 % of stiction is too much. Stiction guarantees cycling and variability. Stiction is much more harmful than the other valve problems. For example, hysteresis is undesirable, but usually not really a problem. Another example is the valve characteristic, which can be compensated by a non-linear function inserted in the controller output or in the positioner.


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Linearity

Linearity

Notes Loop requires characterization or MUST be detuned. The process gain GP varies from 0.283 to 1.96; the ratio of these values is 6.92. This ratio should be under 2 and the process gain values should be between .5 and 3.

Many control loops are difficult to tune because they are non-linear. This means that the process gain changes as a function of the measurement or controller output. Without any linearization, the controller will need to be tuned for the condition when the process gain is the highest. This results in sluggish tuning everywhere else.

Linearizing these loops will improve control since the controller will be better tuned over the complete operating range. With a characterizer, you may be able to get uniform performance across the entire range of your process - so you can run your production at its optimum rather than de-tuning for oscillations.


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Asymmetry

Process Model With Increasing PV Process Model With Decreasing PV

Asymmetry

Notes: Enter Your Asymmetry Description Here

Compare PID tuning or models identified with the ExperTune Process Modeler. Does the process respond differently in the up direction versus the down? If so, can you reduce or eliminate the discrepancy? If not, then you must use the more conservative tuning.

To determine the tuning parameters and the appropriate filter, use the worst case: least aggressive tuning or worst model. The worst model is one with the largest dead time and a highest process gain.


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Test Chosen To Compute Tuning Parameters tsampling : 0.5sec


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Note: This part of the test must represent the worst situation to ensure the loop stability. The worst case is a high process gain GP, a long dead time Td and a small time constant Tc.

Bode Plot

Tuning Parameters


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Tuning Parameters, Performance, And Robustness

Process model found Old and new tuning parameters (fastest response)

Current New

P 50 220

I 35 4.6

D 0 .52

F 0 .026

RRT 17 49

Robustness Performance Indices (From current to new)Performance Increase 73%

Robustness Increase 294%

Valve Travel Index (based on noise simulation) 118%

Valve Reversal Index (based on noise simulation)

-4.59%

Response of the loop to a set-point change Response of the loop to a disturbance or a load change

Parameters

Notes Initial battery of tests


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Loop Summary Table

Notes

Performance Indices

Performance Increase: The Performance Increase shows how much better your plant responds to upsets with the new tuning. Usually the Performance Increase is directly proportional to the money you can save using the new tuning. With poor tuning, an upset in the direction towards expensive results causes you to give away product. Or, a load causes off-spec product. With better tuning you can give away less expensive ingredients while staying on spec.

Robustness Increase: This is the percent that the robustness improves with the new tuning. It is based on comparing the new and current tuning. If the loop is less robust with the new tuning, then the Robustness Increase will be negative. You can use the Robustness Increase as a quick and rough way of assigning a numerical value to the change in robustness from current to new settings. If either loop is unstable, the Robustness Increase will show N/A.

Valve travel and reversal indices: Use these values to predict valve wear by comparing current to new. The less controller output travel and direction changes, the less valve wear. The Valve Travel Index is the percent improvement in controller output travel from current to new. If the new PID tuning parameters (and filter) result in more travel than the current ones, the Valve Travel Index will be negative. The Valve Reversal Index is the percent improvement in controller output reversals from current to new. If the new PID tuning parameters (and filter) result in more reversals than the current ones, the Valve Reversal Index will be negative.

Robustness PlotThere is always a trade-off between tuning the loop tightly, and robustness or sensitivity to the process changing. If tuned too fast, a small change in the process could cause the loop to be unstable. Use the robustness plot to assess this tradeoff. Robustness plots graphically show the tradeoff between tight tuning and stability

The two axis of the plot are process gain and dead time. The plot has a region of stability and a region of instability. The solid (red and blue) lines on the robustness plot are the limit of stability. To the right and above the solid lines (higher gains and dead times) the closed loop process is unstable. To the left and below the solid lines, the closed loop system is stable. There is a cross in the center of the blue line trapezoidal-like figure. This cross is at the actual process gain and dead time. On the graph, moving away from the cross shows what happens when gain and dead time changes. Process gain and controller gain changes have the same effect on closed loop system stability.


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The blue line is for the current PID tuning. The red line is for the new PID parameters. Generally, a safety factor or divisor of 2 is "reasonable" for a loop. These points are represented by the vertices of the blue line "box" in the robustness plot. Use it as a design aid. For practical system stability keep the limit of stability line outside the "box". The vertices are connected by lines that are straight on a log log plot.


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Time Line Analysis

Time Line Analysis

Notes

Time Line Items ValuesSample Interval 0.5Filter Time 0.054Derivative Time 1.1Dead Time 3Equivalent Dead Time 5.3Integral Time 4.1Lag Time 2.3Relative Response Time (RRT) 43

Evaluation Ratios ValuesDead Time / Sample Interval 6Filter Time / Derivative 0.049Controller Integral / Derivative 3.7Filter / Dead Time 0.01


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Evaluated Time Items EvaluationsSample Interval to Process Dead Time

Caution: Better if range greater than 9

PV Filter to Controller Derivative

Excellent

Controller Derivative to Controller Integral

OK

PV Filter to Process Dead Time

OK


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Performance Evaluation

Performance Evaluation

Notes


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Closed Loop Before and After

Before After

Set point change

Statistical analysis


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Histogram

Note :Power Spectral - Before

Note :Power Spectral - After

Note :


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Auto or Cross Correlation - Before

Note :Auto or Cross Correlation - After

Note :


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Conclusions And Recommendations

Enter your conclusions here.


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To Do Valve Hysteresis 0.528 % Gain 0.822

Enter your valve recommendations here.Linearity Loop requires characterization or MUST be detuned..

Asymmetry Enter Your Asymmetry Description Here .PID parameters Initial battery of tests

Conclusions And Recommendations


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OK Equipment performing well

CF Values configured at the plant

MO To be modified

VM Should be checked and analyzed more

RL Should be repaired in the plant

RA To be repaired (during next shut-down)

N/A Not applicable

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Documents

Thursday, July 08, 1999 › SampleReport.doc · Web viewHysteresis Check. Found Acceptable values Hysteresis 0.528 % less than 1% . from 1 % to 2% more than 3% ideal acceptable